It has taken longer than expected. More than 17 years later, what worked in sheep finally appears to be working in humans (see "Insulin-making cells created by Dolly-cloning method"). This is a potentially major medical breakthrough, but no longer feels as challenging as it once did. In fact, after endless hope, hype and failure, it is hard to feel there is anything brave or new in this line of research.

To say stem cell science has a chequered past would be an understatement. Those with long memories will recall that South Korea's fallen stem-cell hero Woo Suk Hwang falsely claimed to have replicated the Dolly technique in humans in 2005.

Controversy has dogged the field ever since. Because the Dolly technique involves the destruction of human embryos, its ethical dimensions have been fiercely debated. But the search for alternatives is not going well.

In January, spirits soared when a Japanese team announced a simple way to create embryonic stem cells with no embryos required. Now, however, the situation is verging on farcical: the results have yet to be verified, the lead author has been accused of misconduct, and the head of the committee that investigated her has resigned over mistakes in his own publications.

Another promising technology that turns adult cells into "induced pluripotent" stem cells has been stalled by concerns that it could lead to cancers and other problems. We will know more when the first human trials start later this year.

It is still too early to say whether the latest breakthrough is the real thing or another false start. But signs are promising: two groups have independently shown that it works, reducing the chance of yet another disappointment.

The promise of stem cell medicine clearly remains alive and well. That is good news. But if we must revert to the techniques of the late 1990s, ethical concerns will return too: embryos, cloned adults and more. We may see that brave new world yet.

This article appeared in print under the headline "Life in the old sheep yet"

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